1. Field of the Invention
The present invention relates to power lifting jacks and in particular to power lifting jacks that have their upward and downward travel distances automatically controlled by a limit switch.
2. Description of the Prior Art
Grain bins, silos and other such structures are constructed through the use of powered lifting jacks. These jacks are attached to the inside of the structure and are synchronously operated to lift an upper section of the structure so that a lower section may be constructed and attached to the underside of the upper section. The jacks are then lowered to set the newly constructed section on the ground and to be detached from the upper section, and are then attached to the newly constructed lower section. The jacks now lift the structure by the newly constructed section so that yet another section can be attached below the section attached to the lifting jacks. This is repeated until a desired height of the bin, silo or other structure is achieved.
It is important for the sake of efficiency and the stability of raising the structure that the operation of the jacks be coordinated, and that the extent of their upward and downward travel be limited. Electric limit switches have been used in some applications to provide these upper and lower limits and to shut off the power to the lifting jacks when either limit is reached.
One type of lifting jack used in the past had a rod fixedly attached to the moving member of the jack, and a limit switch fixedly attached to the non-moving member. Up and down stops on the rod would trip the limit switch when the upper or lower limit of travel of the moving member of the jack had been reached. Several problems, however, have been encountered with this arrangement. First, the rods would occasionally get bent in transportation or from abuse of the equipment and would trip the limit switch prematurely, or not trip it at all. The delay in resetting the limit switches or straightening out the rod can be costly since there is an entire work crew at the construction site. Second, the adjustment of the rods and limit switches was difficult. Third, variation in tolerances of the inner and outer members of the jack could adversely affect the operation of the limit switch. There has been a continuing need for an improved limit switch for use with lifting jacks.